My son had this done last year, at 8 yo. There is no reason it can't be mixed with kool-aid. If your doctor won't do that, ask to have it done at Children's. They always flavor it. I disagree with the comments to have an endoscopy. The barium swallow is much less invasive, and an endoscopy is often not necessary. My son, unfortunately, is going in for the endoscopy anyway, partially due to the fact that medicine is not working, so they need to test for a specific diagnosis that his sister has. Part of why an endoscopy is higher risk is because they take biopsies in an endoscopy, plus they are entering your body, so the risk of infection is much higher.
We have great docs over at Children's if you need referrals, but I'm figuring that's probably where you're going since you live so close. We're 25 miles Southwest of there, and it's still our preference for most of these kinds of things. Kind of looking forward to the new satalite hospital to open, although the GI will still only be at the main hospital. Oh, another plus to the swallow study, there is no anesthetic involved, which, no matter how light, the kids don't like the after feeling, and it can be dangerous for them all day, because it throws their balance off, but they don't feel like it. The last time my daughter had it, she was running around tilted at random times. It was funny to watch, but was a bit scary, because she felt just fine, so wasn't slowing down.
This procedure was really fun for my son. They positioned a screen so that he could see the x-rays as they took them. He thought it was really cool. He even got to see after the barium started entering his intestine. He was pretty impressed, overall, and was sorry for his sister that she only got the endoscopy. For her, the doctors were looking for a very specific diagnosis that can only be confirmed by biopsies, so the Barium swallow was not an option. He also said the barium tasted really good with the koolaid. If I recall right, he even got to pick the flavor he wanted. They also have juice, snacks, and popsicles for afterward.
I hope this make sense. I'm sick today, so probably rambled a lot. I hope it all goes well, and you get some good answers.